Water is used as a coolant in a number of industrial processes as well as in the generation of steam. However, undistilled or untreated water contains a number of impurities which can affect heat transfer, fluid flow or cause corrosion of process equipment. Accordingly, when municipal or untreated water is utilized for cooling or steam formation, the water must be treated with appropriate chemicals in order to inhibit scale formation on industrial equipment.
Typically, the problematic impurities in water are metal cations, such as calcium, barium, magnesium and sodium as well as some anions, such as bicarbonate, carbonate, sulfate, phosphate, silicate and fluoride. When the water contains an excess of these anions and cations precipitates are known to form on equipment surfaces in the form of scales or deposits.
The presence of the scales or deposits adversely affects the rate of heat transfer and therefore the efficiency of the system. Further, the cleaning or removal of such scales or deposits is expensive because it typically requires a shutdown of the system.
A number of chemicals have been provided to reduce or inhibit scale and deposit formation in industrial water systems. More specifically, water-soluble polymers that are anionically charged have proven useful. One particularly useful water-soluble polymer is polyacrylic acid and modified polyacrylic acid; although other water-soluble polymers that are at least partially anionically charged are in use as well.
However, the employment of water-soluble polymers in industrial water systems presents its own set of problems, because the concentration of the polymers in the water must be carefully monitored. For example if too high a concentration of the polymer is employed, then the cost/performance efficiency of the system is adversely affected. In contrast, if too little of the polymer is employed, scaling and/or deposition and/or corrosion will occur. As with other chemical treatment of aqueous systems, there is an optimal concentration that should be maintained.
Methods for determining the concentration of water-soluble polymers in aqueous systems are available. For example, there is a patented fluorometric method for determination of polyelectrolytes using fluorochromatic dyes, see U.S. Pat. No. 5,389,548, which is incorporated by reference.
Other current methods for determining the concentration of water-soluble polymers in aqueous systems rely, in part, upon the formation of turbidity, with such principles of such processes described in the reference, "Turbidity Science", by Michael J. Sader, Technical Information Series--Booklet No. 11, from the HACH Technical Center for Applied Analytical Chemistry. One such method is the TRANSPORT-PLUS.RTM. DR/2000 Procedure from Nalco Chemical Company. The TRANSPORT-PLUS.RTM. DR/2000 Procedure is a multi-step absorbance based turbidimetric method that takes approximately 20 minutes to determine the level of TRANSPORT-PLUS.RTM. polymer.
A colorimetric method is the Hach polyacrylic acid method (Method 8107 from Hach, telephone number (800) 227-4224). The Hach polyacrylic method uses iron thiocyanate chelation to detect polymers with calibrations based on polyacrylic acid products. The known turbidimetric methods and the Hach method suffer from extensive absorption/desorption/washing processing steps with multiple reagents and long reaction times.
There is another colorimetric analytical method recited in U.S. Pat. No. 4,894,346, (hereinafter the '346 patent). The method described in the '346 patent is relatively complicated and difficult to carry out in the field. When conducting the method described in the '346 patent, the pH of the water sample must be adjusted, at the outset, prior to the combination of the sample with the reagent which contains the dye.
A data sheet describing this colorimetric method (based on the European patent application corresponding to the '346 patent) includes in the procedure a waiting period of 30 minutes for completion of the reaction necessary to finish the analysis. Under field conditions thirty minutes is too long a time to wait for the method to be practical. Accordingly, there is a need for an improved colorimetric method for measuring the concentration of water-soluble polymers in aqueous systems that is easier, more accurate, more selective and faster to conduct than the currently available methods.
Yet another disadvantage associated with currently available colorimetric and turbidimetric measuring methods is their intolerance to ions and other contaminants commonly present in municipal and industrial water systems. For example, currently available colorimetric methods such as those described in the '346 patent are susceptible to inaccuracy in highly alkaline systems, highly acidic systems or where the presence of ions or other impurities exceeds certain concentrations. Turbidimetric methods are even more susceptible to inaccuracies caused by the presence of ions and other contaminants then are colorimetric methods. Accordingly, there is a need for an improved method for measuring the concentration of polymers in aqueous systems that is more tolerant of common contaminants and other common properties of municipal and industrial waters.